Vortex ropes in draft tube of a laboratory Kaplan hydro turbine at low load
Application Area 6: Turbomachinery Internal Flow
Application Challenge AC6-15
This test case deals with flow unsteadiness in the draft tube of a laboratory model of a Kaplan turbine operating at an off-design load with only 39% of the nominal flowrate, studied experimentally and by computational modelling and simulations in the Institute of Thermal Physics in Novosibirsk, Russia (Minakov A. etÿal., 2017). The experiments were carried out in a 60:1 scaled-down laboratory model (see figure below), in which the turbine was mimicked by a set of fixed and rotating swirlers, designed to generate the draft-tube entry flow conditions as in a real turbine. The complementing computational studies were performed using several RANS models: the realizable k-? and k-?-SST linear eddy-viscosity models (LEVM), and the basic (LRR) Reynolds-stress model (RSM), then DES (detached-eddy simulations) and LES (large-eddy simulations). The RANS and DES computations were done on numerical grids with 2 and 6 M (million) cells, and LES on 6 and 19.3M grids, the latter serving as the reference fine-grid simulations. The flow patterns, vortical structure and turbulence statistics in the turbine draft tube (DT), their effect on flow stability and pressure pulsations at a low load, appear to be governed by the conspicuous unsteady twin helix ropes. All these features are well reproduced by the LRR RANS model, DES and LES, but to a large extent remained intractable to the considered LEVMs.
Schematic view of the laboratory turbine with its draft tube and a blow-up of the swirler set
Contributed by: A. Minakov [1,2], D. Platonov [1,2], I. Litvinov , S. Shtork , K. Hanjalić  —
 Institute of Thermophysics SB RAS, Novosibirsk, Russia,
 Siberian Federal University, Krasnoyarsk, Russia,
 Delft University of Technology, Chem. Eng. Dept., Holland.